I only have limited knowledge, but here's my thoughts anyhoo:
1) The info for numbers can be obtained from the BCU for the UK, but I'm not sure about the US or Europe. I've seen the figure before for the UK, so its definately out there.
2) I'd say the brands would be very similar. Just because a brand is made in the States doesn't mean it isn't popular here - take Jackson for instance - every other playboat is a JK Star. However, I'm not sure this works both ways, as Pyranha are very much a UK company...
Some kayaking companies include - Robson (german), Liquid Logic (US), Jackson (US), Dagger (EU?), Pyranha (UK), Prijon (EU), Eskimo (German?), Necky, Perception, Wavesport, Riot...
As regards the two markets, I think you've really got to consider the differences in types of kayaking. After spending my holiday in Cornwall I noticed there has been a massive surge of people buying sit-on-top kayaks and taking them out in the surf. It seems like an easy entry point into the kayaking community, as you can basically walk into a shop not knowing anything about kayaking, and come out with everything you need to start. Playboating, or creeking, on the other hand require a certain knowledge of the sport and the necessary equipment required to succeed (e.g. bouyancy, suitable helmets/padding, outfitting - not to mention a grasp of the basic techniques), thus they are not as simple as other lines of kayaking. Then of course there is polo, slalom, marathon kayaks etc etc. Totally different niche markets within the broad umbrella of 'kayaking'.
3) Gotta be careful with this one, as you need to think proportionately to population size. I also doubt that anyone would know this without research... but hey, someone might suprise you.
4) Email some companies such as pyranha (more UK based) and Jackson (US based)... I'm sure they'd be happy to help out.
5) This is a bit easier, as they tend to be slightly different from the kayak manufacturers. Some names to look up are Nookie, Palm, Peakuk, Newton (BAs), Playboater, Yak, Sweet (helmets), Protec (helmets), Werner (paddles), Robson (paddles), Carlisle (paddles)... list goes on. A good idea might be to go on a shop website, such as Kayaks and Paddles, or Brookbank, and go through their product lists.
Most of this info can be found by searching manufacturers sites, or getting in contact with them directly. Although I can't really speak for the US or Europe, I hope I've given you a good starting point to build on!! You've definately got some work ahead of you! Let us know how you get on.
Matt